1. Field of the Invention
The present invention relates generally to the field of corn breeding. In particular, the invention relates to corn plants and seeds comprising bm3 and gt1 genes exhibiting improved growth and composition traits, and derivatives and tissue cultures thereof.
2. Description of Related Art
The goal of field crop breeding is to combine various desirable traits in a single variety/hybrid. Such desirable traits include greater yield, better stalks, better roots, resistance to insecticides, herbicides, pests, and disease, tolerance to heat and drought, reduced time to crop maturity, better agronomic quality, higher nutritional value, and uniformity in germination times, stand establishment, growth rate, maturity, and fruit size.
Corn plants (Zea mays L.) can be bred by both self-pollination and cross-pollination. Both types of pollination involve the corn plant's flowers. Corn has separate male and female flowers on the same plant, located on the tassel and the ear, respectively. Natural pollination occurs in corn when wind blows pollen from the tassels to the silks that protrude from the tops of the ear shoot. Breeding techniques take advantage of a plant's method of pollination. Thus, by controlling pollination process, plant breeding allows to production progeny specifically from selected parent plants.
North American farmers plant tens of millions of acres of corn at the present time and there are extensive national and international commercial corn breeding programs. A variety of naturally occurring mutations are know for various corn varieties, but traits that are agronomically advantageous are often accompanied by other undesirable characteristics. One goal of corn plant breeding, therefore, is the introgression of advantageous genes into an agronomically superior genetic background to produce plants that of greater commercial value.
Decreased corn stock lignin content is a desirable trait in corn crops, since plants with decreased lignin can be used to produce fodder with increased digestibility (Vignols et al. 1995). The brown midrib 3 (bm3) gene, which is mutant version of the COMT gene encoding caffeic acid O-methyltransferase, confers decreased lignin content to corn plants. For instance, silage from corn plants comprising bm3 is a preferred diet for ruminants, and can increase milk yield in dairy cows (Oba et al., 2000). Thus, corn comprising bm3 has greater commercial value than comparable corn varieties lacking bm3. A brown midrib 2 (bm2) gene has also been described (Burnham and Brink, 1932)
Another desirable corn trait is efficient regrowth potential after cutting. In the lower latitudes plants with enhanced regrowth potential can enable a second crop harvest. Alternatively, in northern latitudes where there is a shorter growing season, regrowth of harvested plants can be used as forage for livestock or allow for harvesting of additional material for fodder. Grassy tiller 1 (gt1) is a gene known to alter the phenotype of corn tillers, and to enhance regrowth of plants after cutting. Thus gt1 expressing plants will also have commercial advantages plants lacking the gene.
While the expression of brown midrib or gt1 alone is advantageous a plant comprising both genes is of exceedingly high value. Decreased lignin content and increased regrowth after cutting, conferred by brown midrib and gt1 enables increased production of highly digestible fodder, or forage for live stock. Thus plants comprising both genes can greatly increase the quality and amount of livestock feed that may produced from a given crop field. However, the possibility of breeding such plants regardless of labor was unknown, due to additional complications such as linkage drag and epitasis that occur when attempting to introgress multiple genes into plants. For example, it has been documented that the penetrance and expressivity of the grassy tiller phenotype is variable depending on the genetic background, indicating that successful introgression of the grassy tiller phenotype is unpredictable (Tracy et al., 1982). The combination of the two genes was also unpredictable with respect to the phenotype obtained. Thus, there has been a longstanding but unfilled need in the art for corn plants comprising the gt1 and bm2 or bm3 genes displaying reduced lignin content and increased regrowth after cutting.